1. Field of the Invention
The present invention relates to bandwidth compression apparatus, and more particularly to facsimile data compression apparatus which is adapted to data transmission systems directed at true-false or black-white information.
2. Description of the Prior Art
In data transmission, and particularly in transmission of facsimile information such as information describing the black and white content of a printed page, there is typically much information which is passed describing character detail often beyond the noise level or the printing error level of character definition.
Typically, facsimile transmission is performed by a conventional scanner which selects a section of a page and which transmits by section coordinates data indicative either of the white or the black reflectivity status at those coordinates. The information therefore is in the form of either a black or a white where for example the black can be selected as a binary one and the white can be selected as a binary zero. When received, the data is reassembled according to the same coordinates in order to recreate the characters appearing on the page.
Such data compression techniques are generic both to facsimile transmission as well as transmission of data describing a TV image. In latter applications, the characters do not have a well defined edge and very often include gradations in contrast which is not required in hard edged character facsimile transmission, such as the image of a printed page. One such prior art system is described in U.S. Pat. No. 3,632,867 to Markow and is built around a variable velocity scanner and which further includes in its process a prescan of the page being transmitted in order to find scan groupings indicating white areas. Other prior art systems are exemplified by the patents to Law, U.S. Pat. Nos. 3,541,244; 3,372,228 and 3,463,876, all of which are particularly directed at television image bandwidth reduction and which in detail compress by transmission of alternate scanning coordinates with time. Such a technique, while particularly adapted to television picture transmission, is not effective for single image transmission since only every second pass the full scan field of the television image is transmitted. In this manner, transmission compression is accomplished by what in effect constitutes a real time filtering system, i.e., the display bandpass is in effect reduced by a factor of two and the information normally attenuated in such a generic filter constitutes the compression, or the effective reduction and bandwidth of the prior art transmission systems. The above references are considered typical of the prior art. While in each case these references address themselves to bandwidth compression techniques which are pertinent to the problems specifically dealt with, such techniques are overly complex when applied to facsimile transmission.